Extraction turbine



Nov. l1, 1941. scHwENDNER 2,262,559.

-ExTRAcTIoN TURBINE Filed Oct. 10, '1940 -2 Sheets-Sheet l IU ll Ooo o ATTORNEY Nov. 411, 1941. A, F SQHWENDNER 2,262,559

EXTRACTION TURBINE 2 Sheets-Sheet 2 Filed OCT.. l0, 1940 BY a@ ATTORNEY Patented Nov. 11, 1941 UNITED STATES PATENT OFFICE EXTRACTION TURBINE tion of Pennsylvania Application October 10, 194:0, Serial Nm 360,548

7 Claims.

My invention relates to extraction turbines and it has foran object to provide control mechanismy for the` admission and extraction valves such that both the admission and extraction valves are operated in response to governing and extraction pressure effects. f

A further object of the invention is to provide extraction turbine control mechanism which is not only effective to secure pressure compensation butt which may also be readily modied or adopted f or either speed or load compensation.

.1f-l` further object, of the invention is to provide a turbine having admission and extraction valves and control mechanism for the latter including a governor providing a transformed hydraulic pressure andk means providing controlling fluid pressure. dependent upon extraction steam pressure together with means responsive to transformed and controlling pressures for operating both the admission and the extraction valves.

A further object of the invention is to provide, for a turbine. having two extraction lines and aclmission and extraction valves, a control mechanism embodying a governor providing a transformed hydraulic pressure, a first pressure regulator responsive to steam pressure of the-first eX- traction line to provide a first hydraulic pressure varying inversely as the steam pressure and aA second hydraulic pressure varying directly as the latter, a second pressure regulator responsive to steam` pressure of the second extraction line to provide a'hydraulic pressurev dependent on the extraction steam pressure, together' with servomotors for the valves and controlled by pressureresponsive devices, all of the pressure-responsivedevices having transformed hydraulic pressurev applied thereto for moving the valves in a closing direction, the pressure-responsive device forv the admission valve having said rst hydraulic pressure appliedA thereto for moving the latter in an opening direction, the pressure-responsive device ofl the first extraction valve having said second hydraulic, pressure applied thereto for opening the latter and the pressure-responsive device of they second extraction valve having said third hydraulic pressure applied thereto for opening the latter.

These and other objects are effected by theI invention as will be apparent from the following description and claims taken in connection with the accompanying drawings, forming a part of this application, in which:

Fig.; 1 is a fragmentary view, partly in elevaturbine withthe improved control mechanism applied thereto;

Fig-. 2` is agview similar to Fig. 1, but showing a turbine having` two extraction lines with the improved control mechanism;

' tion valve. I4', and between the turbine sections.

The valves I3 and I4 are moved in opening and, closing directions by servo-motors, at I6 and IIf, respectively.

rEhe servo-motor, at I6, includes operating piston I8 and a pilot valve I9 for controlling the application of motivev iiuid to the latter. Similarly, the servo-motor, at Il; includes an operating piston 20 and a pilotvalve 2I.

The pilot valves I 9 and 2| are normally maintained in aneutral or cut-olf position and are moved by means for applying forces thereto. The follow-up' connections of the pilot valves I9 andv 2I with their respective operating pistons include springs 22 and 23 for exerting forces on the latter which vary to the extent of relative change inl position of the operating pistons with respect tothe pilot valves. pressure-responsive devices, at. 24 and 25, providing for the application of fluid or hydraulic pressure: forces thereto. y

The pressure-responsive device, at 24; includes a pressure or abutment member ZBseparating the pressurechambers 2li and 28 and presenting opposedpressure 'areas 29 and` 30.r to the latter and the pressure-responsive device, at 25, has an abutment member 3| separating the pressure chambers 32 and 33 and presenting the pressure areas 34 and, 35. tothe latter.

' The turbine drives a governor, at 36, which applies a force dependent upon speed to the transformer, at 311, torprovide a transformed hydraulic pressure which is supplied to the pressure-chambers 2l and 32'to exert force on the pressureareas 29 and 34 tending to move the pilot valves IB'and 2I in directions for servo-motor operation to close the admission and extraction valves I3 and Il.

The governor" may employy any suitable meansV providing aforce or impulse'varying as theA square The pilot valves also have of the speed. In Fig. 1, the transformer has such a force applied thereto by the yweights of the governor 36, as more particularly disclosed in the patent to Gottlieb, No. 2,239,602, April 22, 1941. On the other hand, instead of using a force derived from flyweights, an impeller, such as the impeller 89 of Fig. 2, providing a pressure varying as a square of the speed, may be used, such a pressure being applied to a pressure area abutment so as to exert force on the transformer in the same manner as the iyweights. The impeller 80 illustrated in Fig. 2 is more particularly shown and described in the patent to Bryant, No. 2,035,689, March 21, 1936.

As shown in Fig. 1, the transformer, at 31, embodies an operating member 38 having a cup valve 39 and a body structure 40 providing a transformed hydraulic pressure space supplied with liquid from a suitable source by means of an oriiice 4I and having an escape opening 42 boundedby a seat 42a with which the peripheral edge of the cup valve denes variable annular clearance. the operating member 38 to move the cup` valve to restrict the annular clearance. The. transformedpressure space includes a passage 43 communicating with the chamber 21 and with a conduit 44 leading to the chamber 32 to provide for application of ltransformed pressure to the pressure areas to exert forceson the latter tending to secure movement of the valves .I3 and I4 in a closing direction.l

The extraction line I5 has a pressure regulator,

at '45, having an abutment 46 subject to extrac-` tion line steam pressure and movable in response to the latter to move the lever 41 to provide for K oppositely varying forces exerted by the springs 48'and 49 on the cup valves 50 and 5|, the lever 41 being fulcrumed at 41d and having force exerted thereon by the spring 41h in opposition to the" force of extraction steam pressure applied to i the diaphragm or abutment 46.

The cup valve 50 controlsthe discharge of liquid from the rstpressure space dened by the chamber'28 of' the admission valve servo-motor pressure-responsive device andthe interiors of the conduits 52 and 53 supplied by an orifice 54 from suitable :pressure piping 55 supplied with liquid under pressurej by any suitable-means, as by the gear pump 55a in Fig. 1 or the centrifugal pump 55h in Fig. 2. In like manner, the cup valve 5| l controls the'discharge of liquid from a pressure space defined bythe chamber 33 of the Aextraction valve servo-motor pressure-responsive device and 3 the interiors of the communicating conduits 56 g and 51 supplied with liquid from the orifice 58 from the pressure piping.

Force dependent upon speed is exerted on` .The pressure regulator, at 45, therefore, pro- I videsl for oppositely varying hydraulic pressures in saidrst andv second spaces, the pressure in pressure and that in the second space varying di- Assuming an increase in. steampressure, the

l force acting 'on the cup valve 50 would decrease the first space varying inversely as the steam with the result that the hydraulic pressure acting i on the pressure-responsive area 3l) of the admission valve servo-,motor -would `decrease and the transformed hydraulic pressureacting in the oppositey direction would cause operationof the ad- 1 mission; valve servo-motor, at I6, to movevthe admissionvalve I3 in a closing direction.,v Concurrently `.with the decrease in force on the cup valvev 50 4the force acting on-the cup valve 5I` would be increased, whereby the force of pressure acting on the area 35 would be increased and the servo-motor, at I1, would be operated toy move the extraction valve in an opening direction. If the steam pressure acting on the regulator decreases in consequence of increased extraction demand, the opposite operation occurs, the admission valve I3 ybeing moved in an opening direction and the extraction valve I4 'in a closing direction.

From the foregoing, the operation of apparatus so far described will be obvious. With constant extraction demand and change in external load, the latter causes operation of the governor to change the transformed hydraulic pressure in consequence of which the admission valve I3 and the extraction valve I4 are moved together in the same relative directions to change the flow of steam through the turbine suitably to the load. If the load should increase, the speed would tend to decrease and the transformed uid pressure would decrease. The reduction in transformed pressure results in the admission valve I 3 and the extraction valve I4 moving in opening directions until the areas thereof are sufficient to provide` for flow of steam adequate to care for the increase in load. On the contrary, should the load decrease, the Atransformed fluid pressure would increase and the admission and extraction valves would be moved in a closing V-direction to restrict the flow of steam. With constant external load, if the extraction demand increases, the admission valve I3 is moved in an opening direction and the extraction valve I4 is moved in a closing direction, the increased flow through the turbine section Il and the diminished flow through the section I2 satisfying the increase in extraction demand with maintenance of the extraction pressure and the increase in power of the section Il in consequence of the increased steam flow therein being compensated for by the diminished power of the section I2 occasioned by decreased steam flow therein. Should the extraction demand decrease, the contrary operation Would take place, the admission valve moving in a closing direction and the extraction valve moving in an opening direction to maintain the extraction pressure and the loss of power in the section I I being compensated for by the increase in powerof the section I2.

Preferably the governor is arranged for isochronous operation, that is, the 'turbine operates at the same speed for all loads. transformer operating member 38 has a piston area 59 opposite to that provided by the cup vvalve 39, and there is provided means for securin-g delayed application of transformed uid pressure to the pressure area 59, the transformed pressure space or'passage 43 having'a conduit 60 com-- municating therewith and with the pressure chamber 6I for the pressure area 59, the conduit or passage 60 being provided' with a restriction or 'orice'62 and having an accumulator or air bel1f63 communicating with the conduit orv passage Vbetween the restriction or orice and the chamber 6 I.

If there is a change in load, the speed changes there begins a transitionperiod during which the pressure acting`v on the piston area 59 gradually Accordingly, thel changes with further change in transformed pressure until the forces acting on the cup valve 39 and on the piston area 59 are in equilibrium, at which time the governor proper is relieved entirely of any burden of exerting counterbalancing force on the cup valve and the forces due to centrifugal and spring effects will be in equilibrium With the governor in its normal or constant speed position. Thus, a correction is made to the transformed pressure sufficient to further modify the steam admission so as to correct for the change in speed due to the change in load so that the turbine operates at constant speed for the full load range.

This isochronous or speed-correcting feature is more particularly illustrated and described in the patent to Schwendner 2,224,321, December 10, 1940, and in the aforesaid patent to Gottlieb. With this arrangement, as the transformed pressure is adjusted for constant speed at all loads, variations of such pressure because of speed change incident to load change are avoided, with the result that the extraction regulator or regulators are required to exert their effects without having to take into account variations in transformed pressure due to speed change incident to load change, and the operation becomes more certain and accurate and hunting is substantially avoided.

In Fig. 2, there is shown a control mechanism for a turbine, at Illa, having, in addition to the first extraction line Ia and first extraction valve Ma between the first and second turbine sections Ila and |2a, a second extraction line 64 and an extraction valve 65 arranged immediately ahead of the third turbine section 66. So far as the control system for the admission valve I3a and the extraction valve Ica is concerned, it is the same in principle as that already described except for certain differences in the governor and the transformer, as will be hereinafter pointed out.

The second extraction valve 65 is operated by means of a servo-motor, at 61, having a-pilot valve 68 controlled by a pressure-responsive device, at 69, having a movable abutment separating the pressure chambers 'Il and 'l2 and presenting to the latter the opposed pressure areas 'I3 and 14, the area 13 being arranged so that the force of pressure exerted thereon tends to operate the servo-motor, at 6l, to move the extraction valve 65 in a closing direction and the force of pressure exerted on the area 14 tending to effect movement of the valve in an opening direction.

In general, the pressure regulator, at 15, for the second extraction line 64 is similar to that already described except that it has only a single cup valve 16 for controlling the escape of liquid from the pressure space dened by the interiors of the conduits 1l and 18 and the pressure chamber 12 and supplied from a suitable source of liquid under pressure by the orifice 19.

Assuming that the extraction demands of the lines l3a and 64 are constant, if there is a change in load on the turbine, the transformed pressure chambers and all of the valves 13a, Ida, and 65 are adjusted to provide the turbine steam iioW required for the load, the valves all moving in an opening direction in case of increase in load and vice versa.

Assuming that the external load of the turbine remains the same and that the extraction demand of the rst line |511, increases, then the consequent tendency of the first extraction line pressure to drop causes movement of the admission valve l3a in an opening direction and movement of the extraction valve Illa in a closing direction to maintain the first extraction pressure within a narrow range of variation, these movements taking place with maintenance of the external load at its previous value. Due to decreased flow in the second turbine section I2a, the second extraction pressure tends to drop, thereby rendering the second regulator, at 15, effective to move the second extraction valve 65 in a closing direction to keep'the second extraction pressure substantially constant. Movement of the second extraction valve in a closing direction decreases the load carried by the third turbine section 66 and therefore the load carried by the turbine. Consequently, the turbine speed declines and the transformed pressure decreases. As the extraction valves are already set by the pressure regulators, the opening of all of the valves to pick up load lost in the third section and to bring the speed back to its original value will not change the pressure in the extraction lines.

Assuming the extraction demand of the first line to decrease, then the operation is the reverse of that just described, the admission valve |3a closing further and the rst extraction valve I4a opening further to maintain the first extraction pressure, and the second extraction valve 65 opening further, as may be required, to maintain the second extraction pressure. As before, any change in load carried by the third turbine section is followed by change in transformed pressure to adjust all the valves to care for the third section load change and to restore the speed to its former value.

If the extraction demand of the second extraction line 64 should change, then the associated pressure regulator, at 15, maintains the second lextraction pressure, the resulting change in power of the third turbine section 66 being reflected, by means of the governor, at 37a., in compensating power changes in the first and second sections Ila and I2a.

Coming now to the differences in the governor and transformer of Fig. 2 as compared to that of Fig. l, in Fig. 2, a centrifugal effect or pressure is provided by `the impeller 80, the force of such pressure being exerted on the pressure area 6| of the movable abutment 82 engaging the piston 83 of the transformer, at 31a, and being opposed by the force of a suitable spring 84.

The transformer, at 31a, includes a housing structure 85 having a pressure port 86, a drain port 81, and a transformed pressure port 88 communicating with the bore 89 and so arranged, with respect to transformer piston lands 90 and 9|, that inner edges of the latter just lap the pressure and exhaust ports with the piston in neutral position. With an increase in impeller pressure in consequence of decreased turbine load and increased turbine speed, the abutment 82 and the transformer piston 83 move upwardly, whereby the 'pressure port 86 is placed in communication with the transformed pressure port 88 and its communicating transformed pressure space to increase the transformed pressure. If there is an increase in load, the contrary operation takes place, the transformed fluid pressure in the transformed pressure space being decreased.

The transformed uid pressure changes in consequence of load change until the force thereof acting on the piston area 63 of the transformer piston 83 again balances the oppositely-acting force due to impeller pressure. To provide for this, the `transformed fluid pressure space includes` a passage 94 communicating with the pressure chamber 95 for the piston area 93.

The 'transformed fluid pressure space also includes the conduit 91 and branches 98, 99 and |00.leading to the pressure chamber 21a of the admission valve servo-motor, at Ilia, to the pressure chamber 32a of the rst extraction valve servo-motor, at Ha, and to the pressure chamber 1| of the second extraction valve servo-motor, at 6.1, van increase in transformed fluid pressure tending to act on these pressure-responsive delvices to bring about movement of all of the valves in a closing direction and vice versa. The transformed fluid pressure supplied to the pressure chambers 21a and 32a isA opposed by first and sec- .ond fluid pressures provided by the pressure regulator, at 45a, arranged as already described in connection with Fig. 1, the regulator responding to a decrease in extraction pressure in consequence of an increased extraction demand on the line |a to increase the first fluid pressure suppiled to the pressure chamber 28a, of the servomotor, at |6a, to move the admission valve |3al in an opening direction and providing for a decrease in second hydraulic pressure supplied to the pressure chamber 33a to movel the first extraction valve `|4a in a closing direction. If the extraction pressure of the line |5a should increase, then the contrary operation would take place, the diminishing first hydraulic pressure resulting in movement of the admission valve |3a in a closing direction and the increasing second hydraulic pressure moving the first extraction valve Illa in an opening direction.. The second regulator, at 15, provides a third hydraulic pressure for the pressure chamber 12. If the second extraction line steam pressure should increase in consequence of decreased extraction demand, then the third hydraulic pressure changes to` bring about movement of the extraction valve 65 in an opening direction and vice versa.

To provide for isochronous or speed-correcting operation, the body 85 of the transformer governor structure has a passage I 02 communicating with the transformed fluid pressure space and with the pressure chamber |03 for the piston area |04 equal and opposed to the piston area 93; and means, for example, the restriction or orifice |05, and an air bell |06 communicating with the passage between the orifice and the chamber |03, are associated therewith to secure delayed application oftransformed pressure to the piston area |04 in order that, as already described, when a change in load occurs with consequent temporary change in speed and transformed fluid pressure, the transformed uid pressure may be further changed or corrected until the turbine speed is restored to normal or constant value, at which time the force due to impeller pressure and acting on'the transformer piston will be supplanted by that of the delayed transformed fluid pressure acting on the piston area |04, the opposed forcesz acting on the transformer'piston areas 93 and |04, then being in equilibrium, and the force acting on the pressure abutment 82 due to pressure Y supplied by the impeller 80 being equal and opposed to the force of the spring 84 acting thereon. If the multiple extraction point turbine is` the steam 'admission may be compensated to care forvariations in extraction demand at either or ,both extraction points without affecting the -power of the turbine, whereby satisfactory operation without hunting may be secured.

IfV the generator driven by the turbine is connected to `an electrical system, then, because of 4variations, in system frequencyy interruption of the isochronous operation should be provided for and the control mechanism should be arranged for load control to avoid swings on the turbine, `as 'more particularly described and claimed in my application, Serial No. 225,272, filed August 17, 1938J Vand assigned to the Westinghouse Electric & Manufacturing Company. To this end, the passage |02 has a conduit, |08 communicating therewith and supplied with fluid under pressure by a suitable load control device, such as shown in said application. The passage |02 and the conduit |08 have valves |09 and I0, respectively, so that the pressure chamber |03 may be supplied with pressure either under delayed transformed pressure, for isochronous operation, or with load control fluid pressure, for load control operation.

While the invention has been shown in several forms, it will lbe obvious to those skilled in the art 'that it is not so limited, but is susceptible of various other changes and modifications without departing from the spirit thereof, and it is desired, therefore7 that only such limitations shall be placed thereupon as are specifically set forth in the appended claims.

What is claimed is:

1. The combination with a turbine having steam admission and extraction valves, of servomotors including pilot valves for operating the admission and extraction valves, means including a pair of pressure areas for controlling each pilot valve, means operated by the turbine to provide a force dependent upon turbine speed,

means for transforming changes in said force into changes in fluid pressure, means responsive to steam pressure to provide a pair of inverselyrelated fluid pressures, means providing for application of transformed fluid pressure to a pilot valve pressure area of each servo-motor, and means providing for application of the inverselyrelatedV fluid pressures to the remaining pilot valve pressure areas of the servo-motors.

2. The combination with a turbine having steam admission and extraction valves, of means operated by the turbine to provide a force dependent upon turbine speed, means for transforming changes in said force into changes in fluid pressure, means for operating both the admission an'd the extraction valves in response to transformed fluid pressure and to extraction steam pressure, and means for modifying the transformed fluid pressure to compensate for speed change of the turbine due to load change.

3. The combination with a turbine having steam admission and extraction valves arid an extraction line communicating therewith immediately ahead of the extraction valve, of means for developing force dependent upon speed of the turbine, means for transforming changes in said force intofchanges in fluid pressure, a pressure vregulator responsive to change in extraction line steam pressure due to change in extraction demand to provide a first fluid pressure varying inversely and a second fluid pressure varying directly with the change in extraction pressure, servo-motors for operating the admission -and extraction valves and including pilot valves, means for controlling the pilot valves in- -cluding pressure-responsive devices, each pressure-:responsive device including an operating element having opposed `pressure areas, means yfor applying transformed iiuid pressure to one of the pressure areas of each pressure-responsive device, Ameans for applying said first fluid pressure to .the other of said pressure areas of the admission valve .servo-motor pressure-responsive device, and means for applying said second fluid pressure to the other of said pressure areas of the extraction valve servo-motor pressure-responsive device.

f4. The combination with a turbine having steam admission and extraction valves and an extraction line communicating therewith immediately ahead of the extraction valve, of means for developing force dependent upon speed of the turbine, means for transforming changes in said force into changes in fluid pressure, a pressure regulator responsive to change in extraction line steam pressure due to change in extraction demand to provide a first fluid pressure varying inversely and a second fluid pressure varying directly with the change in extraction pressure, servo-motors for operating the admission and extraction valves including pilot valves, means for controlling the pilot valves including pressure-responsive devices, each pressure-responsive device including an operating element having a rst pressure area disposed so that pressure applied thereto is effective to operate its servomotor to move the associated steam valve in a closing direction and a second pressure area disposed so that pressure applied thereto is effective to operate the servo-motor to move the steam valve in an opening direction, means for applying transformed fluid pressure to the first pressure areas of the pressure-responsive devices, means for applying said first iiuid pressure to the second pressure area of the admission valve servomotor pressure-responsive device, and means for applying said second fluid pressure to the second pressure area of the extraction valve Servo-motor pressure-responsive device.

5. The combination With a turbine having steam admission and extraction valves and an extraction line communicating therewith immediately ahead of the extraction valve, of means operated by the turbine and providing a force dependent upon turbine speed, means for transforming changes in said force into changes in uid pressure and providing a space for the latter; said transformer including a movable element having first and second opposed piston areas with the f1rst piston area so disposed that the force of transformed fluid pressure exerted thereon opposes said first force; means for applying fluid under pressure to said second piston area including a passage communicating with the transformed fluid pressure space; means for effecting delayed application of such pressure on the second piston area; a pressure regulator responsive to change in extraction line steam pressure due to change in extraction demand to provide a first iiuld pressure varying inversely and a second fluid pressure varying directly with the change in extraction pressure; servo-motors for operating the admission and extraction valves and including pilot valves; means for controlling the pilot valves including pressure-responsive devices; each pressure-responsive device including an operating element having a iirst pressure area so disposed that pressure applied thereto is effective to operate its servo-motor to move the associated steam valve in a closing direction and a second pressure area so disposed that pressure applied thereto is effective to operate `the servomotor to move the valve in an opening direction; means for applying transformed fluid pressure to said first pressure areas; means for applying said rst fluid pressure to the second pressure area of the admission valve servo-motor pressureresponsive device; and means for applying said second iluid pressure to the extraction -valve servo-motor pressure-responsive device.

6.1The combination with a turbine having a steam admission and first and second extraction valves and first and second extraction lines communicating with the turbine immediately ahead `of the iirst and .second extraction valves, of

means for developing force dependent upon the speed of the turbine, means for transforming changes in said force into changes in uid pressure, a first pressure regulator responsive to change in extraction steam pressure of the first extraction line due to change in extraction demand of the latter to provide a first fluid pressure varying inversely and a second fluid pressure varying directly with the change in extraction pressure, a second pressure regulator responsive to change in extraction steam pressure of the second extraction line to provide a third fluid pressure dependent upon the change in extraction pressure of the latter, servo-motors for operating the admission and extraction valves and including pilot valves, means for controlling the pilot valves including pressure-responsive devices, each pressure-responsive device including an operating element having a first pressure area disposed so that pressure applied thereto is effective to operate its servo-motor to move the associated steam valve in a closing direction and a second pressure area disposed so that pressure applied thereto is effective to operate the servomotor to move the steam valve in an opening direction, means for applying transformed uid pressure to the first pressure area of each pressure-responsive device, means for applying said iirst fluid pressure to the second pressure area of the admission valve servo-motor pressureresponsive device, means for applying said second fluid pressure to the second pressure area of the first extra-ction valve servo-motor pressure-responsive device, and means for applying said third iiuid pressure to the second pressure area of the second extraction valve servo-motor pressure-responsive device.

7. The combination with a turbine having a steam admission and first and second extraction valves and first and second extraction lines communicating with the turbine immediately ahead of the rst and second extraction valves, of means operated by the turbine and providing a force dependent upon turbine speed, means for transforming changes in said force into changes in fluid pressure and providing a space for the latter, said transformer including a movable element having said rst force exerted thereon and having first and second opposed piston areas with the first piston. area so disposed that the force of transformed fluid pressure exerted thereon opposes said first force, means for applying fluid under pressure to said second piston area including a passage communicating with the transformed iiuid pressure space, means for effecting a delayed application of such pressure on the second piston area, a first pressure regulator responsive to change in steam pressure of the first extraction line to provide a rst fluid pressure varying inversely and a second fluid pressure varying directly with change in extraction pressure of the latter, a second pressure regulator responsive to change in steam pressure of the second extraction line to provide a third uid pressure dependent upon the change in extraction pressure of the latter, servo-motors for operatingr the admission and extraction valves and including pilot valves, means for controlling the pilot valves including pressure-responsive devices, each'pressure-responsive device including an operating element having a first pressure area so disposed that pressure applied thereto is eiTective to operate its servo-motor to move the associated steam valve in a closing direction and a second pressure area so disposed that pressure 

